1. Field of the Invention
The invention relates to a production method for a cylinder block and, more particularly, to a cylinder block production method for producing a cylinder block by enclosing a cast-iron cylinder liner within a cylinder block body in a casting process.
2. Description of the Related Art
In view of fuel economy improvement through weight reduction of a cylinder block of an engine, a technology for forming a cylinder block by casting an aluminum alloy cylinder body around a cast-iron cylinder liner provided as an insert has been put into actual use.
However, a problem has been found with an engine incorporating a cylinder block formed by casting an aluminum alloy around a cast-iron cylinder liner. That is, as the engine is operated, a gap develops at an interface between the cylinder liner and the cylinder block body.
If a gap forms between the cylinder block body and the cylinder liner, the heat conductivity comes to vary in a direction of the circumference of the cylinder liner. If the heat conductivity varies in the circumferential direction relative to the cylinder liner, the thermal expansibility of the cylinder liner also varies depending on the position in the circumferential direction. As a result, the cylinder liner does not expand in a truly circular shape. If the cylinder liner forming a cylinder bore surface does not expand in a truly circular shape, the cylinder bore assumes a distorted cylinder shape, and has an increased coefficient of friction with respect to the piston that reciprocates within the cylinder bore. This results in various problems of degradation of the engine in fuel economy, performance, durability, etc., for example, increased consumption of oil, accelerated abrasion of a piston ring, and the like.
The problem of development of a gap between a cylinder liner and a cylinder block body around the cylinder liner is not limited to the case where a cylinder liner is enclosed as a cast insert within an aluminum or aluminum-alloy cylinder block body, but also occurs in cases where a cylinder liner is enclosed as a cast insert within a cylinder block of other kinds of metal. That is, this problem can occur in a case where a cylinder block is produced by casting a cast-iron cylinder block body around a cast-iron cylinder liner.
In order to prevent formation of a gap between the cylinder liner and the cylinder block body of a cylinder block formed by casting the cylinder block body around the cylinder liner, techniques have been proposed which improve the adhesion between the cylinder block body and the cylinder liner by providing asperities, protrusions, etc. on an outer peripheral surface of the cylinder liner, that is, a surface of the cylinder liner that adheres to the cylinder block body.
For example, Japanese Patent Application Laid-Open Publication No. JP-A-58-211550 discloses a “cylinder block formed by casting an aluminum alloy or the like around an outer peripheral surface of a cylinder liner of an iron-based casting, the cylinder block being characterized in that the outer peripheral surface of the cylinder liner is provided integrally with many protrusions that have a tapered shape and preferably have an inclined or curved length is buried within the cylinder block of an aluminum alloy or the like by casting” (claim 1). Regarding the protrusions, the description of an embodiment (in the left lower section of page (2) in Japanese Patent Application Laid-Open Publication No. JP-A-58-211550) states “In the embodiment, the protrusions 3 are protruded from the outer peripheral surface of the cylinder liner, and are curved in the same orientation in a circumferential direction.
It is preferable that the protrusions 3 have a protruded length that is at least about 10% of the wall thickness of the cylinder liner 2. For example, the wall thickness of the liner is 3 mm, and the protruded length of the protrusions is 0.5 mm, and a base portion of each protrusion is 1.0 mm, and a distal end portion of each protrusion is 0.2 mm. The intervals between the protrusions are greater than the size of the base portion of each protrusion, so that the fluidity will not be degraded”. Also described is an example in which the protrusions are formed simultaneously with formation of the cylinder liner through the use of a mold. FIG. 30 shows a perspective view of the embodiment disclosed in Japanese Patent Application Laid-Open Publication No. JP-A-58-211550.
Considering that the wall thickness of the cast-iron cylinder liner is normally about 2 mm, the technique in which protrusions of 0.5 mm in length are provided on the external peripheral surface of the cylinder liner as in the embodiment goes against the reduction of the intervals between cylinder bores, and thus makes it difficult to provide a compact cylinder block. Furthermore, if the protruded length of the protrusions is increased to or above 0.5 mm in order to ensure the formation of the protrusions, it becomes more difficult to provide a compact cylinder block. The presence of the protrusions also incurs a danger of degrading the fluidity in the process of casting around the cylinder liner.
Japanese Patent Application Laid-Open Publication No. JP-A-3-238157 discloses a “production method for a cylinder block of an engine formed by enclosing a cast-iron cylinder liner in a cast-iron body material, the production method for the cylinder block being characterized in that an outer peripheral surface of the cylinder liner is subjected to shot peening so as to activate the surface and form many small protrusions, and then the cylinder liner is enclosed as a cast insert within the cast-iron body material” (claim 1). A similar technique in which a surface of a cylinder liner is roughened by shot blast is disclosed in Japanese Patent Application Laid-Open Publication No. JP-A-10-94867.
The techniques disclosed in Japanese Patent Application Laid-Open Publication No. JP-A-3-238157 and Japanese Patent Application Laid-Open Publication No. JP-A-10-94867 are different from the technique of forming protrusions on a surface of a cylinder liner disclosed in Japanese Patent Application Laid-Open Publication No. JP-A-58-211550, in that the outer peripheral surface of the cylinder liner is subjected to a surface roughing process by shot blast.
However, this surface roughing technique based on shot blast has been found incapable of reliably achieving a sufficient adhesion between the cylinder liner and the cylinder block body. The outer peripheral wall surface of the cylinder liner carries undesired substances deposited thereon, for example, sand (silica sand (SiO2)) used as a lining on an internal surface of a mold during a cylinder molding process, and a mold release agent used on the mold.
The performance of shot blast on the outer peripheral wall surface of the cylinder liner cannot sufficiently remove the sand and the mold release agent deposited on the outer peripheral wall surface of the cylinder liner. In particular, due to the sand provided as a lining on the inner surface of the mold, asperities are formed on the outer peripheral wall surface of the cylinder liner. The sand deposited in dip portions of the rough surface of the cylinder liner cannot be removed by shot blast; moreover, sand may be pushed into dip portions by shot blast.
If the cylinder liner carrying the sand and the mold release agent deposited on the outer peripheral wall surface is enclosed as a cast insert in a cylinder block body, the sand and the mold release agent remaining on the outer peripheral wall surface of the cylinder liner are now present between the cylinder liner and the cylinder block body, so that the strength of adhesion between the cylinder liner and the cylinder block body reduces and becomes insufficient. It has been found that during operation of an engine incorporating a cylinder block in which sand and a mold release agent exist between the cylinder liner and the cylinder block body, a gap forms between the cylinder liner and the cylinder block body.
Furthermore, the shot blast performed on the outer peripheral wall surface of the cylinder liner produces pits and protrusions to a certain degree on the outer peripheral wall surface of the cylinder liner. However, the pits and protrusions do not have distinctive features, but are simple asperities. For example, the pits and protrusions formed by shot blast do not have a feature of curved distal end portions of protrusions. Therefore, the shot blast-formed pits and protrusions do not necessarily achieve sufficient improvement in the adhesion between the cylinder liner and the cylinder block body.